Finish handles for configuring potentials

[pixelzoom]

Handles for configuring potentials are partially done. This issue is to remind me what still needs to be done.

• fix Finite Square width handle for multiple wells (dragBoundsProperty, drag callback)
• fix Poschl-Teller width handle for multiple wells (dragBoundsProperty, drag callback)
• electric field for Finite Square and Poschl-Teller handles
• Finite Square separation handle
• Poschl-Teller spacing handle
• No multitouch
• value labels

Vertical dashed line for:

• Finite Square separation
• Morse width
• Poschl-Teller width
• Poschl-Teller spacing

[veillette]

I have taken a look at the width handle for the multiple wells. The dragBoundsProperty has been generalized. On the other hand the actual calculation of the width is much simpler now. The fix was similar for the FiniteSquare and Poschl-Teller potential.